1. Field of the Invention
The present invention relates to an exhaust pressure-raising device for an internal combustion engine, and more specifically to a technology for achieving both the effect of improving the exhaust emission quality and the combustion stability or the fuel consumption efficiency at the same time.
2. Description of the Related Art
It is known that if exhaust pressure in an exhaust system (from a combustion chamber to an exhaust pipe) is raised by using an exhaust pressure-raising device comprising, for example, an exhaust throttle valve, a reaction of unburnt materials (HC, CO and the like) and oxygen is promoted in the exhaust system, which makes realizable a reduction in emissions of hazardous substances at the time of starting the engine in the cold state and the early activation of catalysts.
In order to obtain the full effect of improving the exhaust emission quality, it is required that the exhaust pressure be set equal to or higher than the prescribed exhaust pressure (800 hPa, for example). Too high exhaust pressure, however, incurs deterioration in the combustion quality, attributable to an increase in the internal EGR and the like, and a decrease in fuel consumption efficiency, attributable to an increase in exhaust resistance. Therefore, the exhaust pressure must be regulated within a prescribed range.
The exhaust pressure, however, is influenced by effective exhaust-channel sectional area (exhaust throttle amount), an exhaust flow rate, and exhaust temperature. Even if the exhaust throttle valve is operated at the same throttle amount, the exhaust pressure varies along with a change in the exhaust flow rate or in exhaust temperature. For that reason, the exhaust pressure-raising device is generally provided with an exhaust pressure-adjusting valve, such as a relief valve, in addition to an exhaust throttle valve, and by using the exhaust pressure-adjusting valve, the exhaust pressure is regulated within the prescribed range.
In general, the exhaust throttle valve is made of a butterfly valve, and the exhaust pressure-adjusting valve is constructed by using a resilient body, such as a spring. This simplifies the construction, thereby improving the mountability of the exhaust pressure-raising device on a vehicle and cutting costs. At the same time, the exhaust pressure is made to be adjustable under favor of the fact that the throttle amount (valve-opening area) corresponding to the exhaust pressure (load) can be obtained due to the properties of the resilient body. For instance, there is a well-known exhaust pressure-raising device with a construction in which an exhaust pressure-adjusting valve is composed of a reed valve formed in the shape of an a plate spring, and the reed valve is attached to a butterfly valve (see Publication No. WO92/00445).
To regulate the exhaust pressure within a prescribed range and to achieve both the combustion stability or the fuel consumption efficiency and the effect of improving the exhaust emission quality at the same time, it is required that the exhaust pressure be maintained substantially constant with regard to the relation of the exhaust flow rate and the exhaust pressure by increasing the exhaust pressure right up to a prescribed value (800 hPa, for example) at least when the exhaust flow rate is equal to a prescribed value (which is a relatively small exhaust flow rate), and by suppressing a rise amount of the exhaust pressure with respect to the exhaust flow rate down to a prescribed amount or less when the exhaust flow rate is equal to or higher than the prescribed value.
To this end, it is considered that a change characteristic of the valve-opening area of the exhaust pressure-adjusting valve with respect to the exhaust flow rate needs to be a prescribed characteristic that fulfills the above requirements.
From this standpoint, the requirements are fully satisfied if a relief valve whose opening is freely adjustable is utilized as an exhaust pressure-adjusting valve.
On the other hand, the relief valve that is freely adjustable has to be operated by an actuator or the like. Moreover, being an exhaust component, the relief valve is required to have high heat resistance, which entails high cost.
On the contrary, as disclosed in the above Publication and the like, utilizing a resilient body makes it possible to actualize an exhaust pressure-adjusting valve with a simple construction at low cost.
A displacement characteristic of a resilient body with respect to the exhaust pressure, however, is automatically determined once materials (resilient modulus) of the resilient body are decided. Consequently, in case that a resilient body is used, the materials of the resilient body uniquely determines the relation of the exhaust flow rate and the valve-opening area, which produces the problem that the degree of freedom thereof is low and that the prescribed characteristics that meets the above requirements cannot be fully obtained.